On your Garmin Edge 830, I don't know if there is any way to modify the figure it shows as a total. However if you create a specific profile for each bike you use then you can see a separate total for each profile. You'll just have to do some math to come up with the actual total for your equipment used prior to establishing the profiles. Use to be able to modify the Garmin device odometer with the old Garmin Training Center software. But that went out of Garmin support about a decade ago. Though I do know it still worked with Win10. But I finally quit using it with Win11. Don't know if a Garmin 830 will be seen by it. But I do think I remember using GTC with a Garmin 820 my son had.

You might have an easier time keeping track of such things with the bike and equipment trackers that RideWithGPS, Strava and others have built into their websites that let you view your ride data from your Garmin, Wahoo or whatever you choose. You can modify the starting mileage with them.